Antimicrobial agents play a crucial role in the treatment of disease. Since the advent of modern antibiotics, it has become apparent that pathogens are capable of developing resistance to antibiotic drug therapy. Increasingly, pathogens that manifest resistance to multiple classes of antibiotics are becoming prevalent, setting the stage for a crisis in global public health. New pharmacological options are urgently needed, preferably including strategies that are likely to remain effective over a sustained course of time.
The incidence of bacterial infections such as methicillin-resistant Staphylococcus aureus (MRSA) causes tens of thousands of deaths annually, and leads to more than $2 billion in health care costs. The discovery of new anti-infective agents has been disappointingly slow. One promising strategy is to develop therapeutic compounds that exert their activity on cellular membranes. Microbial species may be incapable of significantly altering the characteristics of their membrane lipid components. This suggests that compounds capable of selectively disrupting microbial membrane function will yield improved drugs that can deter the emergence of antibiotic resistance.
In the field of peptidomimetics research, extensive efforts have been made to recapitulate the structural features present in naturally occurring bioactive peptides (Ripka et al. Curr. Opin. in Chem. Bio. 1998, 2, 441-452; Steer et al. Curr. Med. Chem. 2002, 9, 811-822; Patch et al. Curr. Opin. In Chem. Biol. 2002, 6, 872-877). Many functional peptidomimetics such as magainin mimics (Liu et al. J. Am. Chem. Soc. 2001, 123, 7553-7559; Wieprecht et al. Biochemistry 1996, 35, 10844-10853; Porter et al. J. Am. Chem. Soc. 2005, 127, 11516-11529; Numao et al. Biol. Pharm. Bull. 1997, 20, 800-804; Rennie et al. J. Ind. Microbiol. Biotechnol. 2005, 32, 296-300), integrin mimics (Pasqualini et al. J. Cell Biol. 1995, 130, 1189-1196; Scarborough et al. Curr. Med. Chem. 1999, 6, 971-981) and somatostatin mimics (Gademann et al. J. Med. Chem. 2001, 44, 2460-2468; Gademann et al. Helv. Chim. Acta 2000, 83, 16-33) highlight the significance of structural mimicry for their function. More recently, efforts have been made to enhance the conformational ordering of peptidomimetic oligomers (Fink et al. J. Am. Chem. Soc. 1998, 120, 4334-4344; Phillips et al. J. Am. Chem. Soc. 2002, 124, 58-66; Abell et al. Lett. Pept. Sci. 2001, 8, 267-272; Clark et al. J. Am. Chem. Soc. 1995, 117, 12364-12365; Dimartino et al. Org. Lett. 2005, 7, 2389-2392). Stabilizing or rigidifying polymer conformations may lead to enhanced binding affinities (Sewald et al., Peptides: Chemistry and Biology. Wiley-VCH: Weinheim, Germany: 2002; Wipf. Chem. Rev. 1995, 95, 2115-2134). To this end, several methods have been developed to enhance the conformational ordering of non-natural polymers (Sewald et al., Peptides: Chemistry and Biology. Wiley-VCH: Weinheim, Germany: 2002; Wipf. Chem. Rev. 1995, 95, 2115-2134; Holub et al. Org. Lett. 2007, 9, 3275-3278). These methods include the introduction of both covalent and non-covalent intramolecular interactions. Some examples of covalent constraints include site-specific macrocyclization via Huisgen 1,3-dipolar cycloaddition (Holub et al. Org. Lett. 2007, 9, 3275-3278), head-to-tail macrocyclization (Gademann et al. Angew. Chem., Int. 1999, 38, 1223-1226; Robinson et al. Bioorg. Med. Chem. 2005, 13, 2055-2064; Wels et al. Bioorg. Med. Chem. Lett. 2005, 15, 287-290; Shankaramma et al. Chem. Commun. 2003, 1842-1843; Locardi et al. J. Am. Chem. Soc. 2001, 123, 8189-8196; Chakraborty et al., J. Org. Chem. 2003, 68, 6257-6263; Angell et al. J. Org. Chem. 2005, 70, 9595-9598; Norgren et al. J. Org. Chem. 2006, 71, 6814-6821; Clark et al. J. Am. Chem. Soc. 1998, 120, 651-656; Yuan et al. J. Am. Chem. Soc. 2004, 126, 11120-11121; Nnanabu et al. Org. Lett 2006, 8, 1259-62; Jiang et al. Org. Lett. 2004, 6, 2985-2988; Mann et al. Org. Lett. 2003, 5, 4567-4570; Wels et al. Org. Lett. 2002, 4, 2173-2176; Bru et al. Tetrahedron Lett. 2005, 46, 7781-7785; Vaz et al. Org. Lett. 2006, 8, 4199-4202; Buttner et al. Chem. Eur. J. 2005, 11, 6145-6158; Royo et al. Tetrahedron Lett. 2002, 43, 2029-2032) and generation of hydrogen bond surrogates via metathesis reactions (Dimartino et al. Org. Lett. 2005, 7, 2389-2392).
Peptoids, for example, are a class of peptidomimetics which comprise N-substituted glycine monomer units (Figliozzi et al, Synthesis of N-substituted glycine peptoid libraries. In Methods Enzymol., Academic Press: 1996; Vol. 267, pp 437-447; Bartlett et al., Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 9367-9371). Peptoids are an important class of sequence-specific peptidomimetics shown to generate diverse biological activities (Patch et al. In Pseudo-peptides in Drug Development; Nielson, P. E., Ed.; Wiley-VCH: Weinheim, Germany, 2004; pp 1-35; Miller et al. Drug Dev. Res. 1995, 35, 20-32; Murphy et al. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 1517-1522; Nguyen et al. Science 1998, 282, 2088-2092; Ng et al. Bioorg. Med. Chem. 1999, 7, 1781-1785; Patch et al. J. Am. Chem. Soc. 2003, 125, 12092-12093; Wender et al. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 13003-13008; Wu et al. Chem. Biol. 2003, 10, 1057-1063; Chongsiriwatana et al. Proc. Natl. Acad. Sci. U.S.S. 2008, 105, 2794-2799). Oligopeptoids can be designed to display chemical moieties analogous to the bioactive peptide side chains while their abiotic backbones provide protection from proteolytic degradation.
Peptoid sequences comprised of bulky chiral side chains have the capacity to adopt a stable helical secondary structure, although some conformational heterogeneity is evident in solution (Armand et al. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4309-4314; Kirshenbaum et al. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4303-4308; Wu et al. J. Am. Chem. Soc. 2003, 125, 13525-13530). The crystal structure of a linear peptoid homopentamer composed of bulky chiral side chains exhibits a helical conformation resembling that of a polyproline type I helix (Armand et al. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4309-4314; Kirshenbaum et al. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 4303-4308; Wu et al. J. Am. Chem. Soc. 2003, 125, 13525-13530). Oligopeptoid sequences incorporating repeating units of two bulky chiral side chains and a cationic side chain form facially amphiphilic helical structures. Recent studies describe antimicrobial activities generated from facially amphiphilic helical peptoids (Patch et al. J. Am. Chem. Soc. 2003, 125, 12092-12093; Chongsiriwatana et al. Proc. Natl. Acad. Sci. U.S.S. 2008, 105, 2794-2799). These peptoid oligomers are reported to be good functional mimics of maganin-2 amide, a peptide antimicrobial agent from Xenopus skin (Patch et al. J. Am. Chem. Soc. 2003, 125, 12092-12093; Zasloff. Proc. Natl. Acad. Sci. USA 1987, 84, 5449-5453).
Antimicrobials can have ‘specific mode of action’ or ‘non-specific mode of action’. Antimicrobials that undergo ‘specific mode of action’ inhibit bacterial metabolism and antimicrobials that undergo ‘non-specific mode of action’ disrupt bacterial membranes (Brogden. Nat. Tev. Microbiol. 2005, 3, 238-250). An example of a peptide antimicrobial that undergoes a ‘specific mode of action’ is penicillin, which inhibits DD-transpeptidase, a bacterial enzyme responsible for cross-linking the peptidoglycan chains that form rigid bacterial cell walls (Waxman et al. Ann. Rev. Biochem. 1983, 52, 825-869). Some examples of peptide antimicrobials that undergo ‘non-specific mode of action’ include maganin 2, protegrin-1, melittin, and alamethicin, all of which disrupt bacterial cell membranes (Waxman et al. Ann. Rev. Biochem. 1983, 52, 825-869).
Amphiphilicity is a common structural feature found in peptide antimicrobials, especially the ones that exhibit helical secondary structure (Tossi et al. Biopolymers 2000, 55, 4-30). There are three widely accepted mechanisms for helical peptide antimicrobials. These antimicrobials are believed to undergo ‘barrel-stave’, ‘carpet’, or ‘toroidal-pore’ mechanisms (Waxman et al. Ann. Rev. Biochem. 1983, 52, 825-869). In all three mechanisms, amphiphilic structure plays a key role in disrupting bacterial membranes.